Digital wireless communication systems have gained widespread adoption in recent years. Common examples of digital wireless communication systems include wireless routers, which are frequently made according to the 802.11 standards, and mobile telephones.
A common problem with digital communication systems is multi-path fading as multiple copies of a signal propagate from a transmitter to a receiver via different paths. For example, one copy of a signal can propagate via a straight line between a transmitter and a receiver and another copy can propagate via a reflection off a structure between the transmitter and the receiver. Because the two copies of the signals are taking different paths, the copies will be out of phase when they reach the receiver. This can result in constructive or destructive interference. As a receiver moves relative to a fixed transmitter, the receiver will either pick up a stronger or weaker sum of the copies of the signal. This variation is fading in the signal.
To address multi-path fading problems, as well as other forms of signal degradation, orthogonal frequency divisional multiplexing (OFDM) has been adopted in many digital wireless systems. OFDM operates by sending digital signals across many different orthogonal subcarriers (or channels). Unlike some other forms of communication which attempt to send a large amount of data over a single carrier at high speed, OFDM spreads the data across multiple subcarriers at lower speeds. This enables OFDM systems to be more robust to interference problems.
Also to improve performance of wireless systems, multiple-input and multiple-output (MIMO) configurations have been adopted, including in OFDM systems. In a typical configuration, a MIMO system may use two or more transmit antennas and two or more receive antennas. By controlling the signals being output on these transmit antennas using block space-time coding techniques, multiple copies of the same data can be transmitted with the hopes that at least one copy will successfully be received by the receiver. One form of space-time coding is Almouti space-time coding.
In a cell configuration of MIMO transmitters, such as with mobile telephones and wireless computer networks, space-time-coded signals from two neighboring transmitters can interfere with each other, even when using Almouti coding. The interference can limit the performance of a receiver on the border between two cells.
Accordingly, it is desirable to provide mechanisms which improve digital wireless communication between two or more neighboring wireless communication cells.